main(){
  //定义一个空字符串
  String? name = null;

  print(name);
  name = "张三";
  print(name);
  // &&
  print(name != null && name.length > 0);
  // ?
  print(name?.length);
  // !
  print(name!);
  // ??
  print(name ?? "李四");
  // ??=
  name ??= "李四";
  print(name);

}